1. Field of the Invention
The present invention relates to a multiple-sensor tracking processing method with reduced latency time. It notably concerns the aircraft radio tracking systems implemented in air traffic management.
2. Discussion on the Background
Air traffic control, or ATC, enables air traffic controllers to ensure the safe, rapid and effective execution of aircraft flights in the air space under surveillance. Its role is mainly to prevent collisions between the aircraft and the ground or vehicles, and in-flight collisions between aircraft. It also consists in speeding up and ordering air traffic, in providing the aircraft with advice and information useful to the safe and effective execution of the flight, such as weather information, information on the status of ground navigation means, traffic information. It finally consists in providing an alert service to warn the appropriate organizations when aircraft need help from emergency and rescue organizations, and to lend these organizations the necessary support.
The data needed to manage air traffic control originate mainly from a plurality of sensors. Among these sensors, primary surveillance radars, or PSR, provide echoes from targets, via azimuth, distance and detection instant information.
Data are also supplied by secondary surveillance radars, or SSR; an SSR sends signals to targets. When these targets are aircraft equipped with transponders, the latter send in return information that is received by the SSR. The information received by the SSR comprises the distance and the azimuth, as well as complementary information supplied by the transponder depending on its type. Thus, a transponder that is compatible with the NC mode supplies an aircraft identification datum (mode A) and a barometric altitude datum (mode C). The set of data supplied by an SSR therefore allows for an identification of the aircraft moving within its field of vision, and the determination of their respective positions in a three-dimensional frame of reference. There are also transponders equipped with a refined mode, or mode S, that supplies, on request, the same data as the NC mode, plus a unique identification of the aircraft coded on 24 bits, and bilateral means of communicating miscellaneous data.
Other aircraft location data and objects on the ground are supplied by multilateration sensors, or MLAT, consisting of a plurality of omnidirectional antennas scattered on the ground, receiving signals sent by an aircraft in order to locate it. These signals can be unsolicited or else sent by the aircraft in response to signals originating from radars. Calculations relating to the differences between the reception times of these signals by at least two antennas make it possible to determine the position of the aircraft. This type of sensor is widely used for monitoring movements on the ground in airport areas and in approach areas. Another technique that works in a similar manner is used to locate aircraft outside airport approach zones, based on the signals received by a plurality of omnidirectional antennas arranged over much wider geographical areas; this technique is named WAM, which stands for Wide Area Multilateration.
There are also surveillance systems that are automatic and dependent on the aircraft, known by the acronyms ADS standing for Automatic Dependent Surveillance. A first ADS system, called ADS-C standing for Automatic Dependent Surveillance-Contract, is used mainly for areas in which few sensors of other types are available, for example desert or ocean areas. The aircraft that are equipped therewith broadcast by radio to a communication satellite data relating to their position, for example determined by an on-board computer on the basis of data supplied by a satellite geolocation receiver or GPS standing for Global Positioning System, and/or by an inertial unit. Other data are broadcast, such as data concerning the route planned for the aircraft, the speeds of the aircraft relative to the air and the ground, weather data (wind strength and direction, temperature, etc.). The broadcasting of these data by the aircraft can be done periodically or in response to certain events, or even in emergency situations. The data are then transmitted by the satellite and are received by a dedicated antenna.
A second ADS system is named ADS-B standing for Automatic Dependent Surveillance-Broadcast. The aircraft that are equipped therewith broadcast by radio the data described previously with reference to ADS-C, directly to a ground antenna. The transmission is done periodically, at a much higher frequency, of at least one transmission per second.
Multiple-sensor tracking, or multiple-sensor radio tracking, is the process which, based on a plurality of detections transmitted by different sensors out of the abovementioned sensors, makes it possible to:                recognize, from the detections available to it, the ones that sample the trajectory of each aircraft present in the detection coverage of each sensor,        reconstruct as accurately as possible the trajectories of the aircraft, that is to say the multiple-radar tracks.        
The expression “multiple-radar tracking”, or “multiple-radar radio tracking”, is used when the sensors are radars, notably of PSR or SSR type.
Multiple-radar tracking is generally based on a technique of merging radar detections, or MPVU which stands for Multiple Plot-Variable Update. With this technique, each point, or “plot”, deriving from the detection of one and the same aircraft is processed as rapidly as possible, in order to update the corresponding multiple-radar track as quickly as possible. There are other known methods of multiple-radar tracking, but among them, MPVU is the one that gives the greatest accuracy. Nevertheless, this accuracy presupposes a relative complexity of the calculations made by the radio-tracking system. A complexity of the calculations leads to a strong latency introduced by the radio tracking system, this latency being able to be defined as the time difference calculated between the moment of output of the information from the radio tracking system, and the moment of reception of the input data.
TIS-B, standing for Traffic Information Service-Broadcast, is a service making it possible to broadcast traffic information to airborne systems, and notably to aircraft pilots. The broadcasting of information is handled by stations on the ground transmitting surveillance information from the ground to the air. TIS-B does not require any transmission of information or acknowledgements of reception of TIS-B messages by the aircraft. On the other hand, for the system to be effective, it is essential for the latency time of the entire information transmission subsystem to be low. The latency time of the entire transmission subsystem comprises the time to be counted from the reception of the information by the various radars and sensors, via the processing of the information by the radars and sensors, the transmission of the information to the air traffic control centre, the processing of the input data at the air traffic control centre, the processing of the data proper by the multiple-sensor radio tracking system, or MSTS standing for Multi-Sensor Tracking System, then the processing of the output data, their broadcasting, and even the reception by the aircraft moving in the space of interest. The maximum acceptable total latency time is defined, for example, in the United States by the standard: DO-286B-TIS-B MASPS (Traffic Information Service-Broadcast-Minimum Aviation System Performance Specifications).
The latency time introduced by a multiple-radar radio tracking system working on the basis of the current techniques is such that, on its own, it contributes in a prohibitive way to the overall latency time of the entire transmission subsystem, making conformity to the abovementioned standard DO-286B-TIS-B MASPS impossible.
Furthermore, the latency times associated with the transmission line from a radar to the ATC centre can be highly variable for a given line, and thus require the tracking systems to use additional buffer memory segments, which extends the latency time introduced by the tracking system; this drawback is known by the name “time disorders”.
Another drawback in the processing of information originating from radars, that has a negative impact on the latency time introduced by the tracking system, is linked to the fact that it is necessary for the system to wait for specific signals from the radars, indicating that all the plots of a given segment have been sent, before beginning the processing on the plots of this segment.